This is the classic philosophical assault on the idea of God being all-good, all-wise, and all-powerful. If a book can answer Hume, it can answer most skeptics today. If it doesn't try to answer Hume, move on to one that does.
Why does God allow evil, particularly atrocities? No book more effectively punctures philosophical and theological abstractions with the sharp end of real life.
Lewis's classic is still the most wide-ranging, accessible, and cogent response to the problem of evil. Don't let its analytical tone make you forget, as many do, that its author lost his mother in childhood and fought on the frontlines of the First World War.
This cri de coeur ("cry from the heart"), rivaled by Nicholas Wolterstorff's Lament for a Son, keeps any intellectual response to evil appropriately modest. Ideas are good; prayers, even angry ones, are better.
The most accessible statement of Plantinga's Free Will Defense, this argument revolutionized the modern philosophical discussion and helped make Christian thinking plausible in the broader academy.